RACHEL SEVILLE
NOTES FROM PRESENTATION GIVEN ON
MARCH 16TH 2026 AT CHESHIRE BONSAI SOCIETY
Acer is the Latin name for maple and there are hundreds of species and cultivars of acer. The two most likely found in the world of bonsai are acer buergerianum (trident maple) and acer palmatum (Japanese maple). This presentation focuses on the latter. There are several cultivars of acer palmatum which are particularly desirable for bonsai. These include Arakawa, Benichidori, Deshojo, Kashima, Matsumurae and Seigen. It also includes the dwarf cultivars Kiyohime, Kotohime and Shishigashira. These cultivars have stood the test of time. They have naturally small leaves and short internodes and they respond well to bonsai techniques and being grown in pots.
Acer palmatum are native to Japan and Korea and they are deciduous trees which grow in the undergrowth or shaded woodland. Their leaves are typically between 4 and 12cm long and are characterised by 5, 7 or 9 sharply pointed lobes. Maples offer many advantages as bonsai. In spring, they exhibit stunning colours in soft, new growth. In summer, they provide vibrant, lush, airy foliage. In autumn, they give glorious autumnal colours and in winter they offer excellent ramified silhouettes. Although aesthetically delicate, maples are horticulturally tough making them great for beginners. They are also suitable for a wide range of styles and sizes.
Spring is the time for repotting and pinching maples. The ideal time to repot is when buds start to swell and leaves start to push through. If repotting is too early, the interior buds may weaken and not extend. Once extension growth occurs, it is too late to repot. As a general rule, young maples can be repotted every 2-3 years and older maples every 3-4 years but it is dependent on the individual tree. Signs that a bonsai needs to be repotted include the tree starting to push out of the pot, the tree starting to lose vigour and condition, roots growing through the drainage holes and soil not draining properly with water running off the surface.
Regarding soil mix, maples in early development can be placed in a standard potting mix. Older maples require a more porous mix. An ideal combination would be a 2:1:1 mix of akadama, lava rock and pumice. Akadama holds moisture and retains fertiliser, lava rock provides aeration and pumice provides drainage. This is different to the standard conifer mix of 1:1:1. Maples pushing new growth require lots of water and they prefer to be on the damp side. Additional akadama helps to facilitate this. These soils are inorganic therefore fertiliser will need to be added.
Spring is also the time for pinching maples. Pinching is where the embryonic shoot between the first pair of leaves is removed. This produces smaller leaves and shorter internodes. The question is whether to pinch each and every maple that we have. For maples in early to mid-development, the primary aim is to develop the trunk, primary branches and secondary branches. They must be allowed to grow freely to achieve this. Leaf size and internode length are not a priority.
There is also the auxin / cytokinin issue. Auxin is a hormone found in the shoots of trees that tells them to grow upwards and outwards. When shoots elongate, auxin is transported towards the roots. Cytokinin is a hormone found in the root system of trees. When shoots elongate, cytokinin is transported towards the terminal shoots. The interaction between auxin and cytokinin is called crosstalk and produces better top growth and rootage. Pinching can limit crosstalk and therefore growth. For younger maples, it is preferable to allow shoots to extend to 4-6 pairs of leaves and then cut back to the first pair of leaves rather than pinching them.
For maples in late development or refinement, embryonic shoots can be pinched as soon as they start to form. The auxin / cytokinin issue is less problematic because abundant top growth is not necessarily desired on mature trees. Pinching should only be carried out on trees in good health. If a tree is weakening over time, it may be necessary to allow it to grow freely for a year to enable crosstalk and recovery.
In spring, maples prefer full sun and a well-lit, airy environment. They like to be damp but not wet. They can be rotated every 2-3 weeks to balance exposure. Fertilisation is dependent on the development stage of the maple. For trees in early to mid-development, feeding should commence as soon as the new buds start to open. A high-nitrogen fertiliser should be used to stimulate maximum growth and build tissue. For trees in late-development or refinement, they should NOT be fed until the first growth has hardened off. A medium-nitrogen fertiliser should be used.
Summer is the time to consider defoliation and POCD as well as leaf thinning and leaf cutting. Defoliation is the complete removal of every single leaf on a tree. The theory is that it will induce a second flush of smaller leaves. This should NOT be carried out on Japanese maples. It is too stressful for the maple, it may produce an inconsistent second flush of growth, it may destroy the energy balance across the tree and over time it may weaken and even kill the tree. If pinching has been applied correctly in spring, it is unnecessary. Partial outer canopy defoliation (POCD) is where all the leaves on the outer canopy of the tree are removed. The theory is that this will allow light and air to penetrate the interior, weaker areas and rebalance growth. This is a technique for trident maples and should NOT be carried out on Japanese maples.
Two techniques that CAN be used on Japanese maples are leaf thinning and leaf cutting. Leaf thinning is where one leaf in every pair is removed across the whole tree. This allows light and air into the interior to preserve the weaker areas and protect fine ramification but it is much less stressful than defoliation techniques. If a maple is still too dense after leaf thinning, leaf cutting can be done. Leaf cutting involves cutting the leaves in half. Like leaf thinning, it allows air and light into the weaker, interior areas of the tree.
These techniques vary slightly when dealing with Arakawa maples and dwarf cultivars like Kiyohime and Kotohime. These maples are not apically dominant. Pinching can be carried out in the lower areas of the tree but the apex should be allowed to fill out. Similarly, leaf thinning can be carried out in the lower branches but the apex should be left alone. With regards to Shishigashira maples, they already have tiny leaves therefore leaf size and internode length are not a concern. Foliage can development into unpleasant bundles if left unchecked. For this reason, every extension should be pruned back to the SECOND PAIR of leaves and then the first pair of leaves should be removed. This will allow light and air into the tree and soften its appearance.
In summer, maples should be placed in partial shade to avoid scorching the leaves. Morning sun and afternoon shade is the preferred combination. Maples can go dormant if the weather is particularly hot. Transpiration can be limited by covering the soil surface with sphagnum moss. It should be remembered that strong winds can damage foliage. For maples in early to mid-development, fertilisation can continue as usual. For maples in late development or refinement, fertiliser can be reduced if the weather is particularly hot and the tree appears to be dormant.
Autumn is the time to think about silhouette refinement. Thick, heavy branches, branches lacking taper and sections with overlong internodes can all be pruned back. However, no major cutbacks should be attempted as temperatures start to fall. This is because no callous will form meaning that wounds will potentially become larger and dry out. If branches are cut right back to the trunk, scarring can occur. It is better to cut back gradually over the course of a year. A stub of up to 2cm (depending on the size of the tree) can be left to prevent dieback and scarring and then removed completely the following spring. Wounds may bleed but this should not be too much of a concern.
It is important to remember that maples are apically dominant except for Arakawa and dwarf varieties like Kiyohime and Kotohime as mentioned earlier. For this reason, the thicker, stronger branches in the apex can be dealt with quite aggressively. Sometimes on mature maples, the apex may need to be rebuilt entirely.
With regard to wiring, it is preferable to use aluminium rather than copper wire. It is softer on maple bark, it is cheaper and it will not be on the tree for very long therefore it is more economical. Wire will bite in much faster on maples than on coniferous trees therefore care needs to be taken to ensure that scar marks are not being created.
In autumn, maples can be placed back in full sun. Sunlight allows maples to create sugars which enables them to produce better autumnal colours. Watering can be increased again to prevent drying out. It is important, however, that maples experience the first cold temperatures of the year because it encourages them to enter winter dormancy. Once their leaves have dropped, they can then enter winter protection. The fertilisation schedule in autumn is exactly the same as that for spring. For trees in early to mid-development, continue with a high-nitrogen feed. For trees in late development and refinement, continue with a medium-nitrogen feed. Feeding can continue until the first frost of the season which in the U.K. is usually around the end of October or beginning of November.
Winter is the time to think about protection and seasonal concerns. Maples must not remain wet because this can cause root rot. Freezing winds can quickly dry maples out and late or early frosts at the beginning of November or end of February can cause fine twig dieback if they are unprotected.
Maples can be left outside over winter. However, if they become frozen, they must remain frozen. Repeated freeze-thaw cycles are very dangerous for bonsai and can kill them. As maples are not active during this period though, there is no benefit to leaving them outside or unprotected. Instead, they can be placed in an unheated shed or garage. If such a space is unavailable, they can be put up against the side of a house. If protected, maples must not get too warm otherwise this will break their winter dormancy and they may start to push out new growth. For this reason, care needs to be taken if placing maples in a greenhouse they may receive sun during daylight hours.
Depending on their placement, maples only need watering once a week or less during the winter months. They do not require any fertilisation during this period but a seaweed tonic can be used once a month if desired.
Finally, growing maples from seed is not recommended unless growing plants for rootstock. This is because the seeds to not necessarily contain the same genetic characteristics as their parent plants. The three methods of propagation used for Japanese maples are air-layering, cuttings and grafting. Grafting is the preferred method of propagation used in bonsai nurseries.
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